Telephone system



Dec. 22, '1942. B F Evws ETL `2,306,067

TELEPHONE sYs TEM Filed Oct. 16, 1940 am n. QN

BfLEw/s aM/rcf/ELL BV @am Patented Dec. 22, 1942 TELEPHONE SYSTEM Benjamin F. Lewis, Bayside, N. Y., and Doren Mitchell, Bound Brook, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of NewYork Application October 16, 1940, Serial No. 361,382

6 Claims.

This invention relates to telephone systems and particularly to the receiving equipment of a transmission line over which alternating currents, preferably within the voice frequency range, are transmitted for effecting the operation of said equipment which controls, in turn, the selective positioning of switching apparatus. More particularly, the invention is in the nature of an improvement in the receiving equipment of the telephone transmission line disclosed in Patent 2,269,022.' granted to G. Hecht and A. A. Lundstrom on January 6, 1942. In said patent is disclosed a telephone system of the type in which automatic switches are controlled by means responsive to alternating currents of different frequencies generated at a subscribers station and transmitted over a line provided with a pluralityv of channel detectors each selectively tuned to a different band of frequencies and responsive thereto to operate a relay which, in combination with a relay operated over another channel detector responsive to another frequency band, serves to function a settable register, or sender, in which is locked in a record of the digit indicated by the two operated relays; that is, by the two frequencies transmitted from the subscriber's station.

In systems of this kind where the signal generating instrument is, usually, a plurality of keys or the like which are operated to produce the different combination of frequencies to which the channel detectors are respectively tuned to respond, it is diicult to keep the transmission line completely free of all frequencies except the signal frequencies generated. From an engineering standpoint it is very desirable to have the signal frequencies Within the voice frequency rangev and, therefore, speech and noise probably will include said signal frequencies. Should speech or noise become impressed upon any sound energy translating device connected to the line such as, for instance, the telephone transmitter and receiver of the telephone station, or should the noise be line noise or contact noise that is, noise generated at the relay contacts included in the built-up transmission line, due to the small variations in the quantity of currents passing through the contacts, then, since the frequency spectrum of speech or noise usually contains components of the different signalfrequencies, the channel detectors, or some of them, are liable to respond to such frequencies and thereby produce a, false registration even though the signal generating instrument may not have been operated.

In accordance with a feature of this invention, the channel detectors are protectedagainst false operation by the use of a guard channel tapped to the `transmission line and responsive to the entire voice spectrum frequency which necessarily includes, of course, the signal frequencies themselves. The guard channel comprises a-band elimination lter adapted to pass all frequencies `intermediate the several different signal frequency bands as Well as all other frequencies on either-.side of the entire signal frequency range, two amplifier detectors of'vwhich one is placed Saiten-the band elimination filter and the other is "tapped to the transmission line after the passband Jlter disposed thereon to pass the signal frequencies only, and a polarized relay interposed lbetweenthe two amplifier detectorsand adapted Analpass-band filter.

-r Thisand other features of the invention will be'more readily ascertained from the following detailed description of said invention, appended claijisv land attached drawing in which is shown schematically certain elements of the telephone system more completely disclosed in the abovementioned patent to G. Hecht and A. A. Lundstrom' and in which certain other elements to which our invention more directly relates, Vare shownin detail.

s Referring, now, to the drawing, A represents a s ubscribers station in an automatictelephone system, said station being equipped with a device I; suitably arranged to produce one or more currents of different frequencies in thelvoice frequency range in response to the depression of keys or the like that represent digits or 4other characters making up the Wanted designation; 2 represents a line extending from the station instrument to the central ofiice whence, through one of a plurality of selectable sender connectors I3, the line is extended to a free sender which comprises coupling transformers 3, 4, 5, 6, 1, 29 and 33, band elimination filter 36 and pass-band filter 31, amplifiers 8, 9, 3| and 32, the polarized relay 34, volume limiter Il, the enabler circuit l0, and the channel detectors fl, f2, f3, f4 andf individually so designated in accordance with the frequency to which each of the channels is respectively assumed adapted to respond, which channels include individually responding relays and the settableV register relays which follow. the operation of the lcliannel relays when operated to be understood that the invention is not limited to channels of this description.

In the operation of the invention, a call is initiated at station A by the removal of the stai tion instrument from the switchhook whereupon the line is then extended by any suitable means to the central office at which an available sender is then connected to said line through the medium of a sender connector I3 after .which key tone is transmitted back to the calling station A by apparatus not shown, .as a signal that the sending device I may be operated to transmit the number of the wanted station, all'Y in the manner described in the above-mentioned patent to A. A. Lungstrom.

When any one of the ten keys of the sending device I is operated, the transmitter and receiver of the station-instrument are short-circuited or otherwise removed from the line in any suitable manner, two out of the five defined frequencies ,fl-f5 are produced in the combination required to designate the digit represented by the operated key, or when currentsv are present in the line produced therein byany means responsive to speech or noise energyor consequent thereto, the same are transmitted over a circuit which may be traced from one side of the sending device I, lower conductor of ,the looped pair 2, lower back contacts of relay I2, next to the inner contacts of sender connector cut-in relay I3, primary winding of transformer 3, inner contacts of relay I3, top outer vback contacts of relay I2, upper conductor of looped pair 2 to the other side of the station sending device I. The current composed of the two.frequencies or produced by the speech or noise, is induced in the secondary winding of transformer 3 and fed into the input side of band-pass filter 31 and band elimation filter 36. Now band-pass filter 31 will pass only five bands of frequencies each of which includes one of the signal frequencies Vthat can be produced by generator device I 'and block out all intermediate frequency bands as well as those on the outer side of the first and theouter side of the fifth of the five signal frequency bands. On the other hand, band elimination filter 36 will pass only the frequencies intermediate each signal frequency band as well as those on the outer side of the first and the outer side of the fifth of the five signal frequency bands but will block out each of the signal frequency bands passed by the band-pass filter 31. Assuming that the incoming signal comprises two signal frequencies only, then the same will be applied from the output side of band-pass filter 31 to amplifier 32 and pad 39. The? portion of the energy which is applied to amplifier 32 is passed, via transformer 29, to the rectifying elel ment 35 and the direct current produced thereby flows through the upper winding of polarized relay 34 over an obvious circuit which includes the winding of this relay. The flow of this current is in such a direction as to move the armature of the relay to its contact A, if it is not already in that position, in which event the signal energy will b-e applied" to. the input side of amplifier 8. The energy .applied to. amplifier 8 will be suitablyA amplifieclifandY applied, 7.5.

through transformer 4, to the input side of the Volume limiter II and is also applied, through transformer 5, to amplifier 3 whence, after suitable amplifcation, it is applied, via transformer 6, to the enabler circuit IU to perform functions discussed hereinafter.

Assume, now, that the incoming current comprises a broad band frequency spectrum such as would be produced, for example, by noise, and that this frequency spectrum includes frequencies within the outside limits of the signal frequency bano, including also the signal frequency bands themeselves as well as other frequencies outside of the limits of the entire signal frequency band. Since band-pass filter 31 blocks all frequencies except those in the signal freqency band, the same will get through to amplifier 32, the latter operating to cause the armature of relay 34 to be operated against its Contact A as already described. The band elimination filter 36, on the other hand, will pass all frequencies except those in the several signal frequency bands. Hence that portion of the entire noise frequency spectrum which does not include signal frequencies will pass through band elimination filter 38 for application to the input side of amplifier 3I when, after suitable amplification, the amplified energy is applied to the primary of transformer 33v to the secondary terminals Vof which is connected the rectifying device 38 to whose midpoint as well as the mid-point of the secondary winding of transformer 33 is connected the lower winding of relay 34, Awound differentially with respect to its upper winding.

The gain of amplifier 3| may be set at a somewhat higherlevel than that of amplifier 32 just to make certain, in the event that the filtered noise energy getting through filter 33 is no greater than the filtered signal energy getting through filter 31, that the quantity of rectified current resulting from the noise and which fiows through the lower winding of relayv 34 is sufficiently large to overcome the rectified current flowing through-the upper winding of said relay in consequence of the filtered frequencies passing through filter 31 andinto the amplifier 32. Since the windings of the relay are opposed to each other, the resultant effect upon the relay is that, due tothe current fiowing through its lower winding, the armature is moved into engagement with contact B,- Ground is now applied by the armature of the relay to the upper input terminal of amplifier 8 and since the lower input terminal is alreadygrounded, said amplifier is shortcrcuited, thus preventing the signal frequencies which'are included in the noise frequency spectrum from being transmitted to the amplifier. In this way the channels are protected from false operation since the signal frequencies present in' the noise spectrum are now passed on to ground on the input side of amplifierv 8 without Aany of them getting through to transformer 4 and hence to the volume limiter I I.

Assuming, now, that the incoming signal comprises the twosignal frequencies only, as would be the case if no noise were upon the line and the frequencies'present would be only those generated by device I, then, as said before, a part of the energy will be applied to amplifier 32 to cause the operation of relay 34 overv its upper winding whereby its armature is held against contact A and amplifier 3 is kept open, and another part would Ibe applied to amplifier 8 where, after suitable ampliiication, the amplified energy isiapplied to the volume limiter Il and, through transformer 5, to amplifier 9.

It should -be remembered that prior to the depression of each key in the generating device I, the transmitter and receiver of the station instrument are exposed to room noises, speech and the like. In such a case the currents generated by the transmitter will, of course, be transmitted over the guard channel, part of said currents going through filter 36 and amplifier 3| to cause rectified current to flow through the lower winding of relay 34 and such components of said currents which comprise signal current components will pass through filter 3'I and amplifier 32 to cause rectified current to flow through the upper winding of relay 34. Since it may be safely assumed that quantity of energy carried by the frequencies other than the signal frequency components thereof is of the same order of magnitude as that carried by the signal frequency components themselves, the armature of relay 34 will engage contact B, in which event amplifier 8 is short-circuited and the signal frequency components of the speech or noise are prevented from passing into the channels. When one of the keys of generating device I is depressed, however, and, in consequence, the transmitter and receiver are disabled, speech and noise can no longer aifect said transmitter and receiver and the frequencies transmitted are only those generated by the operation of the generating device I. Since, in this case, practically no energy fiows into amplifier 3l but practically al1 of it passes through amplier 32, the current through the upper winding of relay 34 will greatly exceed any current that may ow through the lower winding, and the armature of relay 34 will shift from engagement with contact B into engagement with contact A, the short circuit is removed from amplifier 8 and the signal frequencies generated by the operation of device I allowed to pass into the channel detectors.

Considering, now, that portion of the signal energy which passes into the volume limiter II, the energy causes said volume limiter to operate and regulate the energy output in accordance with its adjustment which may be such as to limit the magnitude of output to that required for the satisfactory operation of the channel detectors and no more, regardless of the magnitude of the input energy. The reason for doing this is, of course, to prevent overloading any of the channel detectors which, should it occur in the event of a heavy input signal due, say, to a very short loop 2, would cause false operation of adjoining channel detectors by currents present in the frequency spectrum generated thereby and thus cause the registration of a false number. Hence Iby adjusting the volume limiter to a suitable level of output energy above the operate level of the channel detectors, their false operation upon heavy signal currents is avoided.

The energy output from the volume limiter II is applied, through transformer 'I, to the variable resistance pad I4 whence it is applied through suitable filtering devices to the grids of the detector tubes in the channels. These tubes are normally unresponsive, being under the control of the enabler circuit as described below.

Considering, now, that portion of the signal energy which is transmitted to the enabler circuit l via transformer 6, it will be observed that said enabler comprises a rectifier bridge I6 and four polarized relays I'I, I8, I9 and 20, the armatures of which normally assume the positions indicated in the drawing either in consequence of the current flowing through the polarizing windings as in the case of relays I'I and 28, or in consequence of the currents owing through the lower winding and upper winding as in the case of relays I8 and I9. Further, between each two relays, beginning with relay I'I and ending with relay 20, there is an intermediate condenser-resistance network which joins a contact of the relay on the left with the upper winding of the relay to the right. Considering relays I'I and I8, for instance, the network made up of resistances 22 and 23 and condenser 2I joins the right contact of relay I'I with the upper winding of relay I8. Since the armature of relay I'I is connected to positive battery 30, condenser 2I is normally charged through resistance 22 to the potential of this battery. Condenser 24 of the network between relays I8 and I9, on the other hand, is normally at ground potential since it has ground applied to it through the left contact of relay I8 and resistance 25. Contrariwise, condenser 29 of the network between relays I9 and 28 is charged to the potential of the battery 30 through the upper winding of relay 20 since the short-circuiting ground path for condenser 29 is normally open at the right contact of relay I8, which fact leaves condenser 29 exposed to the charging circuit that traces from the positive pole of battery '39, through the upper winding of relay 20, resista-nce 28 and condenser 29 to ground.

Assuming, therefore, that relays I'I, 8, i9 and 28 have their respective armatures in the positions shown, and that condensers 2|, 24 and 29 are charged, discharged and charged, respectiveiy, as indicated above, ground is normally disconnected from conductor I5 at the right contact of relay 20, which conductor is, in turn, connected to the cathodes of the detector tubes of the sev'- eral channel detectors fl-f5, and causes them to be normally unresponsive to signal energyincoming over the resistance pad I4.

Now when a. portion of the incoming energy is applied to the rectier bridge I6, said energy is f rectified and the current thereof is applied to the upper winding of relay Il. The combined effect of the current now flowing through the upper winding of this relay and that always owing through its lower winding is such as to cause its armature to swing from the right contact into engagement with its left contact. Since condenser 2| is normally charged to the potential of battery 30 through the armature and right contact of relay II, the opening of these contacts immediately causes the condenser to start discharging to ground through resistance 23 and the upper winding of relay I8 to prolong the iiow of current that is normally owing through this relay winding. The direction of the discharge current as well as the normal current through the upper winding of relay I3 opposes that of the current normally flowing through its lower Winding, and since the discharge current is initially of greater intensity through the proper control of the ohmic value of resistance 23, the release of relay I8 is delayed and the armature thereof is caused to break away from its left contact at a time subsequent to the operation of relay Il and to engage its right contact.

During the time that the armature of relay I8 is engaged with its right contact, and remembering that condenser 24 is kept at ground potential while the armature of this relay is making with its left contact, then, as soon as the armature breaks with its left contact, condenser 24 immediately begins.- to chargev over, a circuit traced through resistance 28, theupper vwinding of relay I9 to battery 39, to continue: to maintain the normal current'over this pathrfor a short time subsequent to the operation of rrelay It, whereupon the effect of the currentthrough its lower winding continues ,to be neutralized for a short time by this charging current rather than by the normal current and the armature of relay I9 is thus caused tobreak with its left contact at a time 4subsequent to the operation of relay I8.

At the instant that the armature of relay i9 engages its right contactand before the armature of relay I9 leaves its left contact, a discharge path is completed for condenser 29 which traced from ground through condenser 29, resistance 2l, left contact of relay I9, right -contact of relayitI to ground. The energy accumulated -on condenser 29 as av result of the charging circuit through the upper winding. of relay 29 is quickly drainedoi through the'lowgresistance 2 and a current flow is quickly established in the upper Winding of relay 29 tocause it quicklyto operate and connect ground to conductor I5 to sensitize the detectors, so that When-the armature of relay I9 breaks its left contact, condenser 2 is in an uncharged condition and immediately starts recharging through the upper winding of relay 23 to prolong the current flow therefor a subsequent period. This charging current overcomes the effect of that normally flowing through lower Winding of relay 2U and causes the armature thereof to engage its right contact, thus applying ground to conductor I5 which, as said before, is connected to the cathode of each of the detector tubes in the channel detectors fI--f5. The channel detectors 'are now made responsive and the two which are adapted to respond to the two frequencies in the input current that flows into their respective input side through resistance pad I4 operate. in turn causing the operation of their respective anode relays which, in their turn, cause the operation of relays in the register which designate the digit or character marked by the two frequencies.

It is to be noted that relay i8 will make its right contact at a time subsequent to the energizing of relay Il by the current through the rectifier bridge I6. This time is, of course, determined by the discharge time of the condenser 2| through resistance 23which causes the norma-l current through relay IB to endure for a time (let us say, for example, 20 milliseconds) after the operation of relay Il. At this time relay lil releases and closes its right contact whereby condenser 29 is quickly discharged through the low resistance 2l and establishes a current through the upper Winding of relay 2i! to cause its immediate operation. This, as explained, places ground on conductor i5 to sensitize the detectors at a time 20 milliseconds after the signal was first applied. This delay is for the purpose of allowing the unwanted spectrum ener'gy in adjoining channels to dissipate itself in order to avoid unwanted channel operation from this source. It is the purpose of the condenser 2:5 to maintain, by its discharge current through the upper winding of relay I9 the normal current flowing therethrough to hold relay I9 operated on its left ccntact for a time sufficient to permit the complete discharge of condenser 29 through resistance ZI, left contacts of relay I9 and right contacts of relay I8 to ground. Relay I9 then operates yto break its left contact .and thereafter relay 2i! is maintained on the charging current from battery 30, top Winding of relay 20, 'resistance 28, to condenser 29. When this discharge current has dissipated'itselnrelay 20 lwill operate and break its right Contact .to removeV ground .from conductor I5 regardless of how long thereafter that ,energy is delivered to relay Il by the signal.

'I'he length 'of time during which the armature of relay 20 engages its right contact to cause the channel detectors to remain activateddepends, of course, upon the constants of condensers 24 and 2.13V and resistances 26 and 23, which may be chosen to provide any interval desired. .Most of the time is determined by the charging time of condenser 29 ythrough resistance 28. However, when the condenser 29 is completely charged, the current through the lower windingof relay 29 again becomes effective, the armature of the relay breaks away from its right contact and ground is removed from conductor I5 and, therefore, from the cathodes of the detector element of the several detector channels which, in consequence, will no longer respond to any signal energy Vforthcoming over resistance pad I4 and thereby cause their separate anode relays to release.

rThe circuit conditions prevailing in the enabler during the presence of signal energy in bridge it and after the armature of relay 20 has become disengaged from its right contact, is that relay il will have its armature making with its front contact, relay I8 will have its armature making withy its right contact, While relay I9 Will have its armature on its right contact which will in no Way effect the charge on condenser .29 or the condition of the armature of relay 29 which is out. of engagement with its right contact. Hence it makes no difference how long signal energy prevails upon the line subsequent to the armature cf relay 20 breaking away, from its right contact, said energy Will be ineffectiveto operate the channel detectors inasmuch as ground is disconnected from the cathodes of theirseveral detector elements.

vWhen the signal impulse isr over, relay I1 will releasa reestablishing Y quickly thereby the charging circuit of condenser 2| and the currentthrough the top winding of relay I8. to cause it to make itsleft contact. Relay I8 operates and causes the quick establishment of current through the top Winding of relay I9 to cause it to quickly make its, left contact. In this operation the condenser 29 has not been discharged since ground via theY contacts of .relays I8 and E9 has not been applied. The enabler circuit is now ready for another cycle` of identical .operations upon `the reception of thenextimpulse.

While we have described our invention and the means for lutilizing the. same in connection with its specific application toY a particular kind of. a transmission line, it is to be understood that. various other applications and embodiments thereof may be made by those skilled in the art without departing from the spirit of the invention as deiined within the scope of the appended claims.

What is claimed is:

l. In a telephone system, in combination, a transmission line terminating in aV plurality of channel detectors each adapted to respond to a signal within va definite frequency band transmitted over said line, and a'guar'd channel for said channel detectors connected in parallel with said transmission line comprising means responsive to an energy frequency band including one or morek signal frequencies for preventing said signal frequencies from entering said channel detectors.

2. In a telephone system, in combination, a transmission line terminating in a plurality of channel detectors each adapted to respond to a signal within a definite frequency band transmitted over said line, Iand a guard channel for said channel detectors comprising a first filter adapted to pass signal frequency bands only, a second lter adapted to pass all frequencies excluding those which comprise said signal frequency bands, `and means responsive to energy transmitted over said line including signal frequencies passed through said first filter and other frequencies passed through said second filter for preventing said signal frequencies from entering said channel detectors.

3. In a telephone system, in combination, a transmission hne terminating in a plurality of channel detectors each adapted to respond to a signal within a definite frequency band transmitted over said line, and a guard channel for said channel detectors comprising va first filter adapted to pass signal frequency bands only, an amplifier connected to said first filter, a second filter adapted to pass all frequencies excluding those which comprise said signal frequency bands, another amplifier connected to said second filter, and differentially operated means responsive to energy transmitted over said line including signal frequencies passed through said first filter and amplified by the amplifier connected thereto vand other frequencies passed through said second filter and amplified by said other amplier connected thereto for preventing said signal fre' quencies from entering said channel detectors.

4. In a telephone system, in combination, a transmission line terminating in 'a plurality of channel detectors each adapted to respond to a signal of a definite frequency transmitted over said line, and a guard channel for said channel detectors comprising 'a first filter adapted to pass signal frequency bands only, a first amplifier connected to said first filter, a second filter adapted to pass al1 frequencies excluding those which comprise said signal frequency bands, a second amplifier connected to said second filter, and a polarized relay having one winding coupled to the output of said first amplifier and another winding coupled to the output of said second amplifier, said windings being connected in a differential sense with respect to each other, whereby said relay operates in response to the signal frequency currents passed through said first filter and amplified by the first amplifier connected thereto to allow signal currents to Cil reach said channel detectors and to all other frequency currents passed through said second filter and amplified by the second amplifier connected thereto to prevent said signal frequencies from entering said channel detectors.

5. In la telephone system, in combination, a transmission line terminating in a plurality of channel detectors each adapted to respond to a signal Within a definite frequency band transmitted over said line, and a guard channel for said channel detectors comprising a first filter adapted to pass signal frequency bands only, a first amplifier connected to said first filter, a second filter adapted to pass all frequencies excluding those which comprise said signal frequency bands, a second amplifier connected to said second filter, the gain of said second amplifier being set at a higher level relative to the gain of said Afirst amplifier, a polarized relay having two windings, transformers coupling one winding of said relay with the first amplifier and the other winding with said second amplifier in a differential sense, and rectifying means intermediate each winding and its related coupling transformer whereby the signal frequency currents passed through said first filter and amplified by the first amplifier connected thereto are rectified for passage through one winding of said relay and the other frequency currents passed through said second filter land amplified by the second amplier connected thereto are rectified for passage through the other Winding of said relay for operating said relay on the differential currents to prevent said signal frequencies from entering said channel detectors.

6. In a telephone system, in combination, a transmission line terminating in a plurality of channel detectors each adapted to respond to a signal within a definite frequency band transmitted over said line, and a guard channel for said channel detectors comprising a first filter adapted to pass signal frequency bands only, a second filter adapted to pass all frequencies'except those within said signal frequency bands, and means responsive to energy transmitted over said line when the energy carried by said other frequencies is of the same order of magnitude as that carried by said signal frequencies for preventing said signal frequencies from entering said channel detectors, said means also being operative to permit said signal frequencies to enter said channel detectors when the energy carried by said signal frequencies is much greater than that carried by said other frequencies.

BENJAMIN F. LEWIS. DOREN MITCHELL. 

